Instrumental Rationality Without Separability
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ORIGINAL RESEARCH
Instrumental Rationality Without Separability Johanna Thoma1 Received: 20 February 2018 / Accepted: 4 October 2018 Ó The Author(s) 2018
Abstract This paper argues that instrumental rationality is more permissive than expected utility theory. The most compelling instrumentalist argument in favour of separability, its core requirement, is that agents with non-separable preferences end up badly off by their own lights in some dynamic choice problems. I argue that once we focus on the question of whether agents’ attitudes to uncertain prospects help define their ends in their own right, or instead only assign instrumental value in virtue of the outcomes they may lead to, we see that the argument must fail. Either attitudes to prospects assign non-instrumental value in their own right, in which case we cannot establish the irrationality of the dynamic choice behaviour of agents with non-separable preferences. Or they don’t, in which case agents with non-separable preferences can avoid the problematic choice behaviour without adopting separable preferences.
1 Introduction We make most of our decisions in the context of uncertainty. That is, we don’t know what the consequences of our actions are going to be. What does instrumental rationality require of us in the context of uncertainty? How do we act so as best to achieve our ends? The orthodox answer to this question is that we ought to be expected utility maximizers. Being an expected utility maximizer involves, amongst other things, having preferences over uncertain prospects that are separable: The evaluation of outcomes in distinct states of the world should make independent contributions to the overall assessment of an uncertain prospect. I here want to argue that instrumental rationality does not in fact require separability, and is thus more permissive than expected utility theory. & Johanna Thoma [email protected] 1
Department of Philosophy, Logic and Scientific Method, London School of Economics and Political Science, London, UK
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J. Thoma
There are various counterexamples to separability that show, at the very least, that preferences that violate separability are not obviously instrumentally irrational. I will here focus mostly on what is arguably the most famous, namely Allais’s (1953) paradox. In the light of such counterexamples, we are in need of some compelling argument why it should be instrumentally irrational to have nonseparable preferences. I take the best instrumentalist case that has been made, most notably by Hammond (1988), in favour of separability to consist in an appeal to how agents with non-separable preferences choose in some dynamic choice problems. These agents can be placed in choice situations, the argument goes, where they must choose in a way that is instrumentally criticizable. That is, they end up badly off by their own lights, by making a sure loss, or by behaving in a way that is at odds with their initial assessment of the best course of action. While this a
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